1. Field of the Invention
The present invention relates to a circuit system designed to prevent (the occurrence of a short-circuit mode in a dc motor driving apparatus employing a full-wave rectifier bridge system for converting ac power) from an ac voltage supply to dc power so as to efficiently control the reversing, braking, stopping and other speed changing operations of a dc motor.
2. Description of the Prior Art
Many of known supply voltage ac-to-dc converter circuits for controlling the rotational speed of dc motors employ a half-wave rectifier bridge system which causes no short-circuit mode, so that a part of the upper half or the lower half of an ac waveform is utilized to effect phase control of the speed of a dc motor. There has been another special system of increasing the frequency of a power supply to ensure a stable motor speed, but this system has not been used positively because of the complexity of the circuitry involved. Still other known systems include transistor bridge systems, etc.
In a single-phase half-wave rectifier bridge system, there is a greater variation in the level of the dc voltage across the motor than in the full-wave rectifier bridge system, although no short-circuit mode is caused in the single-phase half-wave rectifier bridge system. As a result, if the dc motor is braked by plugging to control the motor speed or the motor is controlled to operate stably at a low speed, this causes disadvantages of the irregular rotation, the generation of a buzzing noise, etc. These problems can be overcome by means of the full-wave rectifier bridge system. More specifically, in the half-wave rectifier bridge system, each of the thyristors is triggered only once during each cycle of the power supply waveform, whereas, in the full-wave rectifier bridge system each of the thyristors is triggered twice during each cycle with the result that not only such troubles as the irregular rotation, the generation of a buzzing noise, etc., can be prevented but also the controllability of the reversing, braking, stopping and other speeds of the motor can be improved. However, the full-wave rectifier bridge system has not been used positively because of its disadvantage that if any of the thyristors fires erroneously when a dc motor is reversed in rotation, braked or its speed is changed, the short-circuit mode will be established and thus the thyristors will be damaged.
The present invention has been made with a view to overcoming the deficiencies of the above-described ac-to-dc conversion full-wave rectifier bridge system for dc motor driving power supply circuits.